Generally, a circuit generating a bandgap voltage delivers an output voltage in the vicinity of 1.25 volts, near the bandgap value of silicon at the temperature of 0 degrees Kelvin which is equal to 1.22 eV.
In certain circuits, the value of the reference voltage delivered may be adjusted by the value of a resistor or a resistance ratio. One then speaks of an adjustable bandgap reference voltage.
In a general manner, the voltage difference between two PN junctions, for example diodes or bipolar transistors mounted in diode fashion, exhibiting different current densities, makes it possible to generate a current proportional to absolute temperature, generally known by the person skilled in the art by the name “PTAT Current”, where the acronym PTAT stands for “Proportional To Absolute Temperature”.
Moreover, the voltage across the terminals of a diode or of a transistor mounted in diode fashion traversed by a current such as a PTAT current, is a voltage comprising a term inversely proportional to absolute temperature and a second-order term, that is to say varying non-linearly with absolute temperature. Such a voltage is nonetheless designated by the person skilled in the art by the term voltage inversely proportional to absolute temperature and is generally known by the person skilled in the art by the name “CTAT voltage”, where the acronym CTAT stands for “Complementary To Absolute Temperature”. It is then possible to obtain a CTAT current on the basis of this CTAT voltage.
The so-called bandgap reference voltage may then be obtained on the basis of the sum of these two currents through an appropriate choice of the resistors in which these two currents flow, making it possible to cancel the contribution of the temperature factor for a given temperature, so as to render this so-called bandgap voltage independent of the temperature around the given temperature.
An exemplary circuit generating a bandgap reference voltage is described for example in the article by Hironori Banba et al., entitled “A CMOS Bandgap Reference Circuit with Sub-1-V Operation”, IEEE Journal of Solid-State Circuits, Vol. 34, No. 5, May 1999, the relevant teaching of which is incorporated herein by reference.
Such a circuit comprises means for equalizing the voltages across the terminals of a core, comprising a resistor and, in the two branches of the core, two different numbers of diodes, the core then being traversed by an internal current proportional to absolute temperature (PTAT current). Lateral resistors are moreover connected between the terminals of the core and earth, and are then traversed by a current inversely proportional to absolute temperature (Ictat current). An output module is then designed to generate the bandgap output reference voltage.
The operation of the circuit with very low current consumption requires the use of a large resistive value for the lateral resistor generating the current, typically several mega-ohms. Moreover this resistor must be duplicated at each terminal of the core so as to balance the currents. This consequently results in a considerable occupied silicon area.
Another type of circuit delivering a bandgap voltage reference is described in the work by P. R. Gray, P. H. Hurst, S. H. Lewis and R. G. Meyer, entitled “Analysis and Design of Analog Integrated Circuits”, 4th edition, New York: Wiley, Chapter 4, pp. 326-327, the relevant teach of which is incorporated herein by reference. This circuit uses, in particular, cascoded current mirrors disposed between the power supply voltage and the branches of the core, so as to improve the power supply rejection rate. The PTAT current delivered by the core then flows in an additional lateral branch comprising a resistor connected in series with an additional bipolar transistor mounted as an additional diode. This consequently results, across the terminals of this additional resistor, in a potential difference proportional to absolute temperature.
Moreover, the resulting voltage across the terminals of the additional resistor-additional diode assembly is the sum of this voltage proportional to absolute temperature and of the emitter low voltage of the additional bipolar transistor which is, itself, inversely proportional to absolute temperature. An output module makes it possible to deliver a bandgap reference voltage as output.
However, such a circuit exhibits the drawback of requiring a relatively high power supply voltage because of the presence of cascoded current mirrors, stacked between the power supply terminal and the core.